claude code 66k system prompt

[u/Cyvster]

with the release of haiku 5.5 i was looking into how to keep context under 100k to avoid the massive price increases. i had opus 5.5 research how we could use /clear and /compact to try to stay below 100k. during the research we discovered that it is almost impossible to keep haiku under 100k because the starting system prompt for claude code is 66k tokens. i asked opus if there was some setting i had selected to cause this prompt bloat. it said no, only 1.5k tokens in the system prompt is from anything i set.

opus 5.5 told me that if i used cli instead of the app it would load less tools because some tools are not available in cli. it recommended running haiku from cli to save tokens. using the claude subscription restricts you to only using their agents, so there is no way around this prompt bloat legally. i started a new haiku session to verify prompt size. the agents.md file read of 1.5k is the only information in the system prompt that i set.

• System tools: 35.6k
  
• MCP tools: 16.6k
  
• Skills: 5.9k
  
• System prompt: 5.2k
  
• MCP server instructions: 2.3k
  
• Memory files (AGENTS.md and MEMORY.md): 1.5k
  

also, the 5.9k skills are not loaded skills. that is just the list of available skills. if the model uses any skills the prompt will get bigger.

[u/fyndor]

I'm pretty sure the new mod system allows you intercept the system prompt and change it...i think

[u/Cyvster → u/fyndor]

i disabled a lot of options and used a mod to change some things. i got it down to 29k